The present invention relates to a nickel-chromium alloy excellent in a stress corrosion cracking resistance (hereinafter referred to as the SCC resistance), more specifically, to a nickel-chromium alloy in which the stress corrosion cracking resistance is noticeably improved by depositing an insolubilized carbide in grains thereof and by strengthening a coating on the surface thereof.
Heretofore, or tubes, containers and their fittings used in stress corrosion cracking environments including Cl.sup.- ions in nuclear reactors, chemical plants and the like, many nickel based alloys which are considered to be excellent in the stress corrosion cracking resistance have been used. However, it has been reported that even in the case of a 30% Cr-60% Ni system alloy which has generally been used, the occurrence of the stress corrosion cracking cannot be avoided in certain environments.
Thus, an object of the present invention is to provide an alloy which can overcome such a drawback inherent in the 30% Cr-60% Ni system alloy and which is excellent in a corrosion resistance, especially the stress corrosion cracking resistance so that it may be used for the tubes, the containers and their fittings in the nuclear reactors, the chemical plants and the like in the form of thick-walled plates, round rods or pipes.
The inventors of the present case have paid much attention to the fact that the aforesaid 30% Cr-60% Ni based alloy is finally annealed at a relatively high temperature of 980.degree. to 1150.degree. C. in compliance with a carbon content and is used in a state of including no insolubilized carbide, and they have researched into a relation between a morphology of the carbide in the alloy system and its corrosiveness. As a result, it has been found that an active deposition of the carbide, if in the grains thereof, is rather effective for the improvement in the stress corrosion cracking resistance. Further, in view of the report that in environments of a high-temperature water including Cl.sup.- ions, stress corrosion cracks would occur starting from pitting corrosions, the addition of Mo, W and V, which are known as elements effective for the improvement in the pitting corrosion resistance, has been attempted with the intention of strengthening the coating. In consequence, it has been found that the corrosion resistance, i.e. the stress corrosion cracking resistance of the obtained alloy is noticeably improved in cooperation with the aforementioned deposition effect of the carbide, and the present invention has now been achieved.